The present invention relates to a monolithic catalyst, and more particularly, relates to a free-standing or supported monolithic catalyst with micro-scale flow channels and catalyst walls and methods of making such a monolithic catalyst.
Catalytic autothermal reforming of hydrocarbon fuels has been shown to be an attractive way of producing hydrogen for fuel cells. In automotive applications, it is desirable to develop a compact reactor to conserve space and weight. It is also desirable to use monolithic catalysts to avoid particle attrition and pressure gradients common to packed-bed reactors.
State of the art monoliths are based on the use of a metal or ceramic honeycomb support with channel widths as small as 0.5 mm. The active catalyst is typically applied to the monolith using a wash-coat procedure. However, these channel widths are too large and thus the path length that the reactants/products have to travel to/from the active catalyst site is long. Consequently, the process becomes mass transport limited and therefore not very effective for kinetically fast reactions, particularly for automotive applications such as the reforming of methanol or hydrocarbons for the production of hydrogen. Accelerating the mass transport improves the overall reaction rate, which allows processing of more reactants. Thus the volume of the required catalyst is reduced and leads to more compact reactors.
Microreactors have been shown to decrease the diffusion length and maintain good flow characteristics. Microreactors have been demonstrated in a large number of chemical processes including hydrocarbon fuel reforming. Microreactors are characterized by flow channel dimensions that range from the sub-micro to sub-millimeter scale. Microreactors are normally fabricated using micro-technology methods including lithography, wet chemical etching, laser ablation, micro-molding, and advanced mechanical cutting, milling and drilling.
U.S. Pat. No. 5,611,214 discloses a microcomponent sheet architecture where microscale unit processes are performed by microscale components. The sheet architecture includes a single laminate with a plurality of separate microcomponent sections or the sheet architecture includes a plurality of laminates with one or more microcomponent sections on each laminate.
An example of a microreactor fuel reformer, based on U.S. Pat. No. 5,611,214, has been demonstrated. This microreactor is fabricated by cutting channels into stainless steel sheets by either conventional machining or electrodischarge machining. The active catalyst powder is packed into the channels of the plate. These fabrication methods are expensive in terms of equipment and time. In addition, the benefit of the microchannel is lost since the active catalyst forms a packed bed, which increases the pressure drop. The metal substrate adds weight and volume to the reactor.
It is desirable to reduce or eliminate the problems associated with known reactors. The catalyst industry generally uses simple and inexpensive methods to process a wide variety of catalyst materials. It is desirable to develop an economical method to fabricate monolithic catalysts with micro-scale flow channels.
A principal object of the present invention is to provide a monolithic catalyst with micro-scale flow channels and methods of making such a monolithic catalyst. Other important objects of the present invention are to provide such methods and monolithic catalyst substantially without negative effect; and that overcome some disadvantages of prior art arrangements.
In brief, a monolithic catalyst with micro-scale flow channels and methods of making such a monolithic catalyst are provided. The monolithic catalyst includes a plurality of thin catalyst walls. The walls having a set thickness in a range from 1 to 150 xcexcm. The thin catalyst walls define a plurality of flow channels. The flow channels are formed by a fugitive material. The flow channels have a set width in a range from 1 to 200 xcexcm.
A monolithic catalyst of the invention is formed by making a flexible strip including a layer of catalyst material and a fugitive layer. The flexible strip allows strips to be cut and formed into selected shapes as needed for a particular reactor design. For example, strips can be rolled into a spiral cylinder or folded into a planar stack. The flow channels are formed by an organic fugitive material, which burns off during processing.
In accordance with features of the invention, using the thin catalyst walls and flow channels having a set width in a range from 1 to 200 xcexcm, provides a reduced diffusion path length that molecules travel between the bulk gas and the active site. Accelerating the mass transport thus improves the overall reaction rate, which allows processing of more reactants. Thus, the volume of the required catalyst is reduced, allowing more compact reactors. Fabrication methods of the invention involve simple, low-cost and scaleable procedures, allowing the flow channel and catalyst dimensions to be easily scaled to a requisite size for a given application. One fabrication method involves tape casting successive layers of fugitive and catalyst materials, and then firing to remove the organic binders and partially sinter the catalyst particles. The slurries can also be cast into thin layers using various processes, including screen printing, wet spraying and spin casting. Another fabrication method for fabricating a supported catalyst involves dipping a pre-shaped metal foil into a slurry containing an active catalyst powder, allowing the slurry to coat the foil evenly and allowing the catalyst slurry coated foil to dry. The catalyst slurry coated foil is dipped into a solution to form a fugitive coating, and the coated metal foil is cut into strips and formed into a selected shape.